Capacitive pressure sensors are used in industrial metrology to measure pressures. For example, pressure sensors designated as semiconductor sensors or sensor chips, which may be produced cost-effectively in a wafer structure using processes known from semiconductor technology, are used as pressure sensors. Pressure sensors designed as absolute or relative pressure sensors normally have a measurement membrane applied onto a base body with the inclusion of a pressure chamber, the outside of which measurement membrane is charged with a pressure to be measured in a measurement operation. Absolute pressure sensors measure the pressure acting upon the measurement membrane relative to a vacuum prevailing in the pressure chamber. Relative pressure sensors measure the pressure relative to a reference pressure supplied to the pressure chamber, e.g., the current atmospheric pressure.
Pressure sensors designed as differential pressure sensors normally have two base bodies, between which the measurement membrane is arranged. In these sensors, a pressure chamber included under the measurement membrane is also respectively provided in each of the two base bodies. In measurement operation, the first side of the measurement membrane is charged with the first pressure via a recess in the first base body, and the second side of the measurement membrane is charged with the second pressure via a recess in the second base body.
Capacitive pressure sensors comprise at least one capacitive, electromechanical transducer that detects a deflection of the measurement membrane dependent upon the pressure acting upon the measurement membrane, and that transduces the deflection into an electrical signal reflecting the pressure to be measured. Semiconductor sensors normally have a conductive measurement membrane that, together with an electrode integrated into the base body and electrically insulated from the measurement membrane, forms a capacitor having a capacitance dependent upon the pressure to be measured.
A differential pressure sensor designed as a capacitive differential pressure sensor is described in German patent, DE 103 93 943 B3. This comprises a measurement membrane mounted between a first and a second base body, the measurement membrane being connected so as to be pressure-sealed with each of the base bodies, with respective inclusion of a pressure chamber, the first side of the measurement membrane being able to be charged with a first pressure via a recess in the first base body, and the second side of the measurement membrane being able to be charged with the second pressure via a recess in the second base body. The base bodies respectively comprise an electrically conductive layer facing away from the membrane and an electrically conductive layer facing toward the membrane, and an insulation layer arranged between the two layers and insulating both layers from one another. Provided in the layer of the base body that faces toward the membrane is a respective electrode spaced apart from the measurement membrane, which electrode, together with the measurement membrane, forms a capacitor having a capacitance that varies as a function of the pressure acting upon the measurement membrane. For this, the electrodes are electrically insulated via a trench from an outer edge region of the respective membrane-facing layer, said edge region being connected with the measurement membrane.
In principle, the pressure difference may be determined using each of the two measured capacitances C1, C2. However, the pressure difference determination preferably takes place using, not the individually measured capacitances, but rather a differential change fin the two capacitances C1, C2. For example, the differential change f may be determined as a product of a constant k and a difference in the reciprocal values of the capacitances C1, C2, according to: f=k(1/C1−1/C2), and exhibits a linear dependency upon the pressure difference to be measured.
With capacitive pressure sensors, a problem exists that a respective capacitive coupling exists, not only between the region of the measurement membrane that deforms according to pressure and the electrodes situated opposite this, but also between the electrodes and their surroundings and between the measurement membrane and its surroundings. Accordingly, in addition to the capacitance that varies according to pressure, a capacitance measured between the measurement membrane and one of the electrodes also includes parasitic capacitances due to the capacitive couplings relative to the environment. The greater the parasitic capacitances in comparison to the capacitance changes of the pressure-dependent capacitance, the changes being dependent upon the pressure-dependent deflection of the measurement membrane that is to be metrologically detected, the smaller the measurement effect, and, therefore, also the smaller the achievable measurement precision.
Moreover, parasitic capacitances lead to nonlinear effects that hinder the determination of the pressure to be measured using the measured capacitances. In particular, parasitic capacitances in differential pressure sensors produce nonlinear dependencies of the differential change f on the pressure difference that is to be measured, the dependencies being dependent upon the size of the differential pressure sensors. Moreover, non-reproducible changes in parasitic capacitances may lead to a falsification of the capacitance measurement signals.
To reduce the negative influences of parasitic capacitances, DE 103 93 943 B3 describes establishing contact between the electrodes integrated into the base bodies respectively through the layer facing away from the membrane and the insulation layer of the respective base body, and shielding the electrodes from the environment of the differential pressure sensor, in that a reference potential is applied to the measurement membrane, the edge regions of the membrane-facing layers, and the layers facing away from the membrane, via an electrically conductive coating applied onto the outside of the differential pressure sensor. For this, the coating is preferably grounded. Alternatively, an electrical circuit connected to the aforementioned modules is described that keeps all of these modules at a ground potential or a reference potential of the connected circuit. However, coating the outsides of pressure sensors produced in a wafer structure is only possible subsequently, after the individualization of the pressure sensors. The coating of every single pressure sensor is complicated and, in comparison to processes that can be cost-effectively executed on the wafer structure, less precise.
Given that the cited components are set to the same electrical potential, a shielding of the pressure sensor relative to the environment of the pressure sensor, similar to that with a Faraday cage, is produced, and the potential relationships in the immediate environment of the electrode inside the pressure sensor are kept stable. However, that inevitably has the consequence that the electrodes and their connection lines running across the respective base body are at an electrode potential, varying over time as a function of the pressure to be measured, that differs from the potential of the modules surrounding it and varies relative to these. Due to the capacitive couplings existing between the electrodes and their connection lines relative to the modules surrounding these within the differential pressure sensor, any variation in the electrode potential produces charge shifts in the immediate environment of the electrodes and their connection lines that immediately retroact upon a measurement signal tapped via the electrode terminals. They thus lead to a falsification of the measurement signal and, therefore, negatively affect the measurement precision.